Passive Methods for the Fast Seismic Characterization of Structures: The Case of Silea Bridge

Abstract

This paper provides useful information about a passive method, usually applied on soils, for defining the frequencies of most infrastructures. Today, the eigen-frequency determination is one of the most significant and binding requirement aspects, especially in relation to the recent earthquakes in Italy (Accumoli, Norcia 2016; Finale Emilia 2012; L’Aquila 2009). The development of inexpensive, fast and reliable procedures to define the eigen-frequency of construction in general and of infrastructures in particular, is the aim of modern civil engineering. Currently, experimental tests are widely applied to evaluate the dynamic behavior of bridges. While the natural frequencies of a structure can be determined using different methods, passive methods are attractive because of their low costs and easy and fast procedures. In this paper, two passive methods, called standard spectral ratio (SSR) and horizontal to vertical spectral ratio (HVSR), are introduced and analyzed. The two methods are applied to estimate the frequencies in the transversal direction of a composite steel-concrete viaduct recently built in Italy. The comparison of the results obtained from SSR and HVSR and a finite element model confirms the possibility of applying the two methods for the dynamic characterization of bridges. In particular, the SSR method provides a correct estimation of the lower order natural frequencies and their degree of amplification. HVSR, which is typically used only on soil studies, provides a reliable early estimation of the frequency of a structure, if the latter is flexible, compared to the soil characteristics. The HVSR method is directly applied to the viaduct, so that an analogy is created between the soil layer and the structural elements. This paper is intended to show how the HVSR method typically applied to soil can reach good results in the dynamic characterization of Silea steel-concrete viaduct.